The present invention relates to an improved process and apparatus for forming coke and especially graphite coke.
Processes for forming coke from petroleum hydrocarbons are well known. See, for example, U.S. Pat. Nos. 3,745,110 and 3,836,434, the disclosures of which are incorporated herein by reference. Such processes involve heating certain petroleum hydrocarbon streams to elevated temperatures, for example 925.degree. to 975.degree. F., and rapidly running the hot hydrocarbons into a relatively quiescent chamber known as a "coking drum." As the hydrocarbons are charged into the coking drum, they undergo coking, i.e., they change state from a liquid to an extremely viscous plastic semi-solid.
When charging of the coking drum with hydrocarbon is completed, it is customary to introduce steam into the bottom of the coking drum. This procedure, referred to as "steam stripping" drives off non-coked hydrocarbons, i.e., portions of the hydrocarbon feed which have not coked. The steam stripping period also allows time for coking of the most recently charged hydrocarbon. In addition, steam stripping provides some cooling of the very hot mass of coke in the coking drum.
After steam stripping, the coke is cooled to a relatively low temperature, i.e., about 200.degree. F. or less, so that it can be safely removed from the coking drum. This is accomplished by charging water into the bottom of the coking drum. During early stages of water cooling water charged into the coking drum is immediately converted into steam which may build to dangerously high pressures. Therefore, care must be taken to adjust the water flow rate during water cooling to prevent high pressures from developing in the coking drum.
When the water cooling operation is completed, the coking drum is ready for emptying. This is accomplished by removing covering plates at the top and bottom of the coking drum called "heads" and breaking up the hardened coke into chunks. Break-up of the coke is normally accomplished by means of high pressure water drills which direct jets of high pressure water into the coke and thereby break up the coke into chunks or pieces. The chunks of coke so formed fall through the bottom of the coking drum into railroad cars or other suitable conveyors for conveyance to calciners or other buyers.
Coke which is removed from coking drums is referred to as "green coke" and still contains molecules which will "crack" at elevated temperatures. Customarily, green coke is subjected to calcination at elevated temperature to cause these reactions to occur and hence complete the coking operation, thereby producing finished petroleum coke.
One of the widest uses for petroleum coke is in the manufacture of carbon electrodes for use in the manufacture of steel and aluminum. Carbon electrodes used for the manufacture of aluminum can be made from most types of coke. However, carbon electrodes for use in the steel industry normally must be made from graphite coke, which is a special type of coke characterized by having a needle-like quasi crystalline structure and which is made from petroleum streams rich in aromatics and substantially free of asphaltines. As is known, it is possible to produce carbon electrodes having co-efficients of thermal expansion of 4.0.times.10.sup.-7 /.degree. C. or less if graphite coke is used as a raw material. However, if non-graphite coke is used the electrodes will have co-efficients of thermal expansion on the order of 6to 10.times.10.sup.-7 /.degree. C. Since it is necessary for electrodes used in the steel industry to have low co-efficients of thermal expansion, only those electrodes made from graphite coke are acceptable to the steel industry.
Unfortunately, the use of graphite coke as a starting material in the manufacture of carbon electrodes will not always guarantee that the carbon electrodes produced have co-efficients of thermal expansion of less than 4.0.times.10.sup.-7 /.degree. C. In this conection, it has been found that some carbon electrodes produced from graphite carbon and otherwise correctly processed have co-efficients of thermal expansion significantly above 4.0.times.10.sup.-7 /.degree. C. This phenomenon is believed due to the character or quality of the graphite-based green coke which is calcined and then processed into the carbon electrodes. However, at the present time there is no reliable analytical procedure which can be used to determine if a particular batch of graphite-based green coke is of acceptable quality and character.
Accordingly, it is an object of the present invention to provide an improved technique for manufacturing graphite-based green coke which will form carbon electrodes having a lower co-efficient of thermal expansion than current practice, preferably a co-efficient of thermal expansion of 4.0.times.10.sup.-7 /.degree. C. or less for a higher percentage of on-stream operations.